The present invention relates to a method and system for the extinction or “killing” of an offshore oil well after an explosion or a blowout causing an appearance of an uncontrolled fountain of oil fluids mixed with gas from the remaining part of the well. The term “oil well” is used herein to describe a well that produces any type of hydrocarbons including oil and gas, but which may also produce a gas condensate or water as part of the fluid mixture discharge that comes out of the well. The present invention more specifically relates to methods for controlling the fluid discharge by gradually decreasing fluid flow using a series of flow restrictor inserts.
In the field of offshore oil drilling, the oil wells are kept under control by means of a column of mud which provides a hydrostatic load sufficient for maintaining overpressure between the well and the external pressure at controlled values. This column of mud, also known as primary well control barrier, is present both inside the well and also in a pipe called riser which connects the drilling platform to the sea bottom.
At the sea bottom, moreover, in correspondence with the well heads, there are generally secondary well control devices, called blowout preventers or BOP, which act as valves which are configured to close the well in the case of uncontrolled discharges of fluids from the well itself.
Often during drilling or well exploration in gas and oil wells, a gas kick may enter into the well space. Gas exits the well reservoir and reaches the bottom hole of the well. If this is not detected immediately, this creates a gas bubble (gas kick) in the hole. Gas kick, according to Archimedes' principle begins to ascend within the annular space of the well. If not allowed to expand, it brings its initial high pressure equal to the formation pressure to the head of the well. At the same time, the pressure everywhere along the well begins to rise. If the BOP is closed, and there is no “washing” in the well, a hydrofracture of formation may occur. As a result, the drilling fluid enters the formation, and the well is filled with gas. If the drill pipe has no check valve, the gas also fills drill pipes up to the wellhead. This may cause a gas explosion that may result in human casualties, environmental pollution and the creation of an uncontrolled fountain. This uncontrolled fountain is very difficult to suppress, because the wellhead is under enormous pressure. As offshore drilling on the continental shelves is progressing into deeper and deeper waters, the problem is many times more complicated when the explosion occurs in deep waters. Suppressing such a well and cleaning of the environment may cost billions of dollars.
Presently known are various techniques for reestablishing the control of the well in case of a blowout, such as for example the techniques of bridging, capping, production of a relief well and assembling a string of pipes for the injecting cement down the well, such string is sometimes referred to as a killing string.
A killing intervention consists of the insertion of a specific string of pipes inside a blowout well. When inserted in the well, the killing string allows conventional killing techniques to be applied such as the circulation of heavy mud, closure by means of inflatable packers, and so forth. This method has proved to be the most rapid, but it can currently only be used in the case of well blowouts in shallow water, i.e. less than 1,000 m. In addition, in order to allow for the adequate flow of cement through the killing string, its internal diameter has to be sufficiently large such as at least 10 cm or more. Inserting such a large string of pipes presents a challenge due to an enormous pressure in the well urging the killing string out of the well.
To date, no practical equipment is available to the industry for the purpose of regaining the control of a deep water abandoned wellhead on the offshore seabed after a blowout causing spilling of reservoir fluids into the sea. The environmental pollution caused by such outpouring of reservoir liquids and gases can have disastrous consequences, as witnessed the recent pollution created over a large section of the Gulf of Mexico and adjacent beaches by the erupted well off the coast of Mexico.
There is a need for an improved method for killing of an uncontrolled fountain from an oil well following a blowout event.